Aim: To determine the purity of aspirin, using five salicylic acid standards of known

volume and measured the absorbance of each by the method spectrophotometric analysis.

Abstract:

In determining the purity of aspirin, five salicylic acid standards comprising of

acetylsalicylic acid and iron (III) chloride of known volume were created and the

absorbance of each was measured. The concentration of each standard was

calculated and a Beer’s Law plot was generated for Absorbance (A) vs.

Concentration (M). The concentration of the reacted salicylic acid in the aspirin

was found from the graph to be (2.81x10-04 M) and this value was used to

determine the % purity of the aspirin tablet as is shown from the calculations to be

95.1%. From this value, the aspirin tablet is considered to be pure.

Introduction:

In a solution, aspirin itself does not absorb light in the visible range. However,

when it is converted to an iron (III) salicylate complex, it absorbs light in the

visible range. The absorption characteristics of this colored solution can be used

to determine quantitatively how much aspirin is in the solution. The intensity of

the color is directly related to the concentration of aspirin present; therefore,

spectrophotometric analysis can be used to determine the purity of aprin. A series

of solutions with different aspirin concentrations can be prepared and complexed.

The absorbance of each solution will be measured and a calibration curve can be

constructed. Using the standard curve, the amount of aspirin in a commercial

aspirin product can be determined.

When conditions are basic, aspirin reacts with water to form the salicylate dianion

according to the equation below:

If the salicylate dianion is mixed with an acidic solution such as FeCl3, a highly

colored (violet) iron III salicylate complex will be produced:

A violet color of the complex will result due to the fact that the complex strongly

absorbs green light. Once this green light is removed from normal white light, a

violet colour can be observed. The absorption of green light can be used to

quantitatively determine the amount of aspirin that is present in the solution. The

more green light that is absorbed, the more violet the solution will be, and

therefore the more aspirin will be present. If green light of a wavelength 530nm

is shown into the solution containing this aspirin complex, some of the green

light will be absorbed while the residue will be transmitted. The intensity of the

absorbed green light leaving the sample, I, will be less than the original intensity

of the green light, I0. The difference between I and I0 is called the transmittance

(T) of the sample and can be defined as T = I/I0.

Absorbance ( A) of the solution can be defined as

A= log (I0/I) = log (1/T). In a typical experiment, several solutions of known

concentration of the complex are prepared. These are standard solutions and the

absorbance of each standard solution is measured at the wavelength of the

maximum absorption (530nm) using a spectrophotometer. A graph of these

absorbance values versus the concentration of each of the standards should yield a

straight line. This relationship is known as Beer’s Law: A = ξ.c.l

Procedure: As outlined in analytical chemistry lab manual by Robert Johnson, pages 45-

47.

Results:

Table 1 showing the mass of the watch glass and acetylsalicylic acid used

Mass of watch glass/g 90.4693g

Mass of watch glass + sample (acetylsalicylic acid)/g

90.6693g

Mass of sample/g 0.2000g

Table 2 showing the mass of mass of the watch glass aspirin used

Mass of watch glass /g 94.8934

Mass of watch glass + aspirin/g 95.0967

Mass of watch glass /g 0.2033

Table 3 showing the absorbance sand concentrations of acetylsalicylic acid and aspirin iron (III) chloride solution at wavelength of 530nm

Calculations:

Molarity of stock soln = moles of solute/volume of solvent

Molar mass of salicylic acid (C7H6O3) = 138.09 g/mol

Moles of salicylic acid = mass of salicylic acid /molar mass of salicylic acid

= 0.2000g of salicylic acid *1 mol of salicylic acid/138.09g of salicylic acid

= 1.45x10-3mols of salicylic acid.

Molarity of stock soln = moles of solute/volume of solvent

= 1.45x10-3mols of salicylic acid/0.500L of solvent

= 2.9x10 -3 M

Molarity of standard soln (A) = M1V1=M2V2

Flask Volume/mL Concentration Absorbance (A) @ 530nm

A 10.0 5.8 x 10 -4 M 0.675B 8.00 4.67 x 10 -4 M 0.527C 6.00 3.5 x 10 -4 M 0.412D 4.00 2.92 x 10 -4 M 0.277E 2.00 1.17 x 10 -4 M 0.139

My Aspirin 5.00 0.301

= 2.9x10-3 M*0.01L = M2*0.05L

M2 = (2.92x10-3 M*0.01L)/0.05L

M2 = 2.92x10-5M/0.05L

M2 = 5.8x10 -4 M

Molarity of standard soln (B) = M1V1=M2V2

= 2.9x10-3 M*0.08L = M2*0.05L

M2 = (2.92x10-3 M*0.08L)/0.05L

M2 = 2.34x10-4M/0.05L

M2 = 4.67x10 -3 M

Molarity of standard soln (C) = M1V1=M2V2

= 2.92x10-3 M*0.06L = M2*0.05L

M2 = (2.92x10-3 M*0.06L)/0.05L

M2 = 1.75x10-4M/0.05L

M2 = 3.50x10 -3 M

Molarity of standard soln (D) = M1V1=M2V2

= 2.92x10-3 M*0.04L = M2*0.05L

M2 = (2.92x10-3 M*0.05L)/0.05L

M2 = 1.46x10-4M/0.05L

M2 = 2.92x10 -3 M

Molarity of standard soln (E) = M1V1=M2V2

= 2.92x10-3 M*0.02L = M2*0.05L

M2 = (2.92x10-3 M*0.02L)/0.05L

M2 = 5.84x10-5M/0.05L

M2 = 1.17x10 -3 M

0.00E+00 1.00E-04 2.00E-04 3.00E-04 4.00E-04 5.00E-04 6.00E-04 7.00E-040

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0

E

D

C

B

Af(x) = 1264.80675538561 x − 0.000990239239264334R² = 0.99847520662283

Beer's Law Plot

Concentration, M

Abso

rban

ce, A

Figure 1. Graph showing Absorbance (A) vs Concentration, M

Molarity of Aspirin soln = moles of solute/volume of solvent

Molar mass of salicylic acid (C7H6O3) = 138.09 g/mol

Moles of salicylic acid = mass of salicylic acid /molar mass of salicylic acid

= 0.2033g of salicylic acid *1 mol of salicylic acid/138.09g of salicylic acid

= 1.47x10-3mols of salicylic acid.

Molarity of Aspirin soln = moles of solute/volume of solvent

= 1.47x10-3mols of salicylic acid in Aspirin/0.500L of solvent

= 2.95x10 -3 M of Aspirin solution

Molarity of 5mL Aspirin soln (E) = M1V1=M2V2

= 2.95x10-03 M*5.00mL = M2*50.0mL

M2 = (2.95x10-03 M*5.00mL)/50.0mL

M2 = 1.475x10-02M/50.0

M2 = 2.95x10 -04 M

From the graph y = 1264.8x-0.001

Molarity of ‘My Aspirin’ stock solution = 2.95x10-04M

Absorbance of ‘My Aspirin’ = 0.354 = y

Concentration of unreacted salicylic acid = x, therefore:

0.354= 1264.8x – 0.001

x = (0.354+ 0.001)/1264.8

= 0.355/1264.8

= 0.000281 M

= 2.81x10 -4 M

Therefore the concentration of the reacted salicylic acid is 2.81x10-04 M. However, we

need the percent (%) purity of the aspirin, therefore:

% of reacted salicylic acid in ‘My Aspirin’ = (2.81x10-04 M/2.95x10-04M) * 100

= 0.951 * 100

= 95.1% of reacted salicylic acid.

Therefore, % purity of ‘My Aspirin’ = 95.1%

Discussion:

A spectrophotometer operates by passing a beam of light through a sample and

measuring the intensity of light exiting and reaching a detector. The beam of light

consists of a stream of photons. When a photon encounters an analyte light,

thereby reducing the intensity of the light beam. In order to determine the purity

of aspirin, using five salicylic acid standards of known volume and measured the

absorbance of each by the method spectrophotometric analysis. The concentration

of each standard and a Beer’s Law plot was generated for Absorbance (A) vs.

Concentration, M. The concentration of reacted salicylic acid in the aspirin was

found from the graph (2.81x10-04 M) and this value was used to determine the %

purity of the aspirin tablet as is shown on the calculations to be 95.1%. From this

value, the aspirin tablet is pure.

Conclusion:

The purity of the aspirin tablet was found from the process of spectrophotometry

to be 95.1% and it can be deduced that the aspirin is pure.

References:

1. Harris, D.C. "Quantitative Chemical Analysis; 8th Edition"; W.H.

Freeman and Company: New York.

2. Johnson, Robert (MSc.), (2010). Analytical Chemistry: Laboratory Manual,

University of Technology Faculty of Science and Sport, Kingston Jamaica.